The present invention is directed to an optical transmission system. More specifically, the present invention is directed to a bidirectional optical transmission system in which sub-carrier multiplexing is employed.
It is known how to provide bi-directional communication systems in the optical fiber environment. An example of one such known configuration is illustrated in FIG. 1. In this configuration, xe2x80x9cbi-directionxe2x80x9d refers to the fact that information can be sent between point A and point B in either direction. However, the system actually consists of two uni-directional transmission systems combined together. In particular, a first optical fiber 120 carries information from transmitter 130 at location B to receiver 100 at location A. This first uni-directional transmission system is combined with a second uni-directional transmission system that employs a second optical fiber 121 to carry transmissions from transmitter 110 at location A to receiver 131 at location B. The obvious problem with this configuration is the need for two separate optical fibers to carry the information between points A and B. It is desirable to provide information between the locations over a single optical fiber if possible.
It is also known in the prior art how to provide bi-directional transmission over a single optical fiber using only one laser transmitter (xe2x80x9cObservation of Coherent Rayleigh Noise in Single-Source Bidirectional Optical Fiber Systemsxe2x80x9d, Wood et al., Journal of Lightwave Technology, Vol. 6, No. 2, February 1988). An example of this configuration is illustrated in FIG. 2A. Here, a laser transmitter 200 and a receiver 210 are positioned at location C. The laser transmitter transmits an optical signal over optical fiber 220 to a second location D. Modulator/receiver 230 receives the information signal from the optical fiber 220 and then modulates the carrier signal received and sends it back along the same optical fiber 220. A splitter 240 then permits a receiver 210 at location C to receive the modulated returned optical carrier that contains information being transmitted from location D. Thus, there is xe2x80x9ctransmissionxe2x80x9d from both locations over a single optical fiber. However, only a single laser transmitter is provided and hence, all of the communications that occur over the optical fiber rely upon the same optical carrier.
It is also known how to provide a sub-carrier multiplexed transmission to create multiple channels with a single optical carrier. Such a configuration is described in xe2x80x9cSub-carrier Multiplexing for Multi-Access Light Wave Networksxe2x80x9d by T. E. Darcie Journal of Light Wave Technology, Volume LT-5 No. 8, Aug. 18, 1987, pages 1103-1110. The article describes a network that increases the usage of an optical fiber transmission system. As described, it is possible to modulate an optical carrier signal with one or more microwave frequency subcarriers, each of which can carry unique data. As the article describes, each access point in a network could be assigned its own subcarrier channel for communication, and be capable of transmitting at that sub-carrier microwave frequency or receiving at that subcarrier frequency. When transmitting from a central location to many users as shown in FIG. 2B multiple sub-carriers f1 to fN can be modulated onto a single optical carrier xcex1 at transmitter 205, thereby expanding the capacity of the optical fiber to serve multiple access points. Each receiver 2351, to 235N is adapted to receive information from one of the N sub-carriers. Also, each transmitter 2501, to 250N transmits back to receiver 215 using a sub-carrier frequency. When transmitting from the users to the central receiver 215 a phenomenon known as xe2x80x9coptical beat-interferencexe2x80x9d can cause severe system impairments (as described in xe2x80x9cOptical Interference in Light Wave Subcarrier Multiplexing Systems employing Multiple Optical Carriers.xe2x80x9d By C. Desem, Electronics Letters, 7th January, 1988, Volume 24 No. 1, pages 50-52).
In one proposed bidirectional transmission system it has been discovered that optical beat interference exists even when the same optical carrier is not used in both directions.
In view of this optical interference problem and the short comings of the prior art systems, it is desirable to provide a truly bi-directional transmission system over single optical fiber which avoids the problem of optical beat interference.
The present invention provides the desired bi-directional transmission system. The present invention achieves the bi-directional transmission capability with reduction or avoidance of optical beat interference by providing a unique combination of transmitters and receivers at the access points of the network.
In accordance with an embodiment of the present invention, the transmitters at two different locations along the single optical fiber produce transmission signals in accordance with two different optical spectra. The optical spectral characteristics for the two transmitters are selected so as to assure that the wavelength of the optical carriers are different during the operation of the system. This avoids the production of optical beat interference. The selection is made so as to assure that even as the wavelength of the transmitters may vary based on certain stimuli such as temperature (such variation also being referred to as drift), the wavelengths will have a very low probability of overlapping thereby insuring a reduction or avoidance of optical beat interference.
In one of the embodiments the optical carriers for the two transmitters are specifically selected to have optical frequencies which differ by more than the maximum frequency employed as a sub-carrier (fmaxsc). The optical carrier""s wavelength is equal to the speed of light xe2x80x9ccxe2x80x9d divided by the optical frequency f (xcex=c/f). Therefore, this sets a specification on how the wavelengths of the optical carriers must differ (|xcex94xcex|=xcex94f/C xcex2). This requirement must be increased to account for the laser linewidth (variations in the optical frequency due to noise), chirp (variations in the optical frequency due to modulation of the optical carrier), and drift. Temperature controllers could be provided with these transmitters so as to stabilize the lasers to avoid drift. However, if the wavelength difference is sufficient, then even without temperature control the effects of optical beat interference should be minimized.
In a multiple frequency laser (also known as a multimode laser), such a Fabry-Perot laser, the optical carriers must be selected so that each optical frequency of one carrier differs from all of the optical frequencies of the other carrier by fmaxsc.
In another embodiment the characteristics of the optical spectra differ in that the mode spacing (the difference in wavelength between two frequencies of one multiple frequency laser) of the two lasers differ sufficiently that even with drift the optical spectra of the two lasers will never coincide at all of the optical frequencies. This difference in mode spacing can be achieved either by using two Fabry-Perot lasers of different lengths or by employing a Fabry-Perot laser and a single frequency laser such as a distributed feedback laser.
By selecting the optical spectral characteristics of the two transmitters to differ sufficiently, the present invention assures that bi-directional transmission in the sub-carrier multiplexing environment is attainable.
Further details regarding the invention will be described below.